RF power amplifiers of conventional design typically suffer from a significant efficiency reduction when the output power level is adjusted to values below the peak design output power by varying the input drive to the power amplifier. This is true for all classes of amplifiers: A, B, C, E, and F. Maintaining efficiency with RF output power cutback is an important requirement for radios that are designed to save battery power as a result of reduced power operation. Generally speaking, the power efficiency of power amplifiers operated at small signal levels will be poor unless the amplifier incorporates features expressly to remedy that incipient deficiency.
One known option for varying output power while maintaining efficiency is to adjust the voltage supply to the amplifier stage. However, this option is inconvenient since a high efficiency DC/DC (direct current) converter is required in the case of a battery powered RF amplifier.
Another option is to use one or more variations on the well-known Doherty Amplifier which comprises a carrier amplifier and a peaking amplifier. This option is relatively expensive when embodied as an integrated circuit due to the types of circuit required.
A further option is described in U.S. Pat. No. 5,276,912 issued to Siwiak et al., wherein a multi-mode transformation network is used to vary an impedance presented at an output transistor as a means to vary power level. Typically such implementations suffer from lossy components such as PIN diodes in the multi-mode transformation network. Also, the output impedance at the output transistor is not at the optimal value for the power level when operated in the lower power mode. Moreover, all impedance mismatches have negative implications for voltage levels arising out of reflections thus giving rise to consequent reliability and longevity issues especially where the amplifier is implemented as an integrated circuit such as an analog CMOS (complementary metal-oxide semiconductor) circuit.
Thus there is a need for an improved amplifier design capable of multiple power levels and a superior tradeoff between power efficiency and cost.